This used to drive me insane, not being able to center an image on the photo paper.... One day out of curiosity I had wondered what would happen if I didn't touch the margins but rather just entered the "Cell Size" well... all my images are centered on the paper now. YMMV, but that's what works for me, currently I use LR4.1 Obviously I'm sure there are times when one wants a larger margin on the left than right, bottom than top, I don't seem to have a problem with those, just centering caused me grieve. If you want to print borderless, you have to set the margins to 0. Ironic part is setting the cell size is fewer steps....
Vance